This invention relates to drive transmissions for powered vehicles, and more particularly, to a fluid pressure operated control system for shifting such a transmission between drive conditions.
In general, transmissions for vehicle use include a number of clutches or brakes which are actuated by fluid pressure supplied thereto to establish selected drive conditions within the transmission in response to movement of a vehicle operator's shift lever.
Smooth, efficient shifting requires careful control and coordination of the pressure changes at the several clutches or brakes. This, in turn, requires a fairly complex pressure modulating system between the source of operating fluid and the selector valve which directs such fluid to the appropriate clutches.
For optimum performance and safety, a transmission control system should also provide certain functions in addition to such pressure modulation. For example, it can be hazardous to the operator and equipment if the vehicle should be started while the transmission controls are positioned in other than the neutral position.
Toward this end, it is desirable to provide a safety device which prevents actuation of the mechanisms of the transmission when fluid pressure is first supplied to the system unless the shift control lever is in neutral, or until such time as the shift control lever is momentarily returned to neutral.
Patents of interest in this area include U.S. Pat. No. 3,596,536 to Starling, and U.S. Pat. No. 3,468,194 to Horsch et al (both assigned to the assignee of this invention). While such systems as disclosed in those patents have proved to be relatively effective in operation, it will be understood that it is always desirable to vary such systems so as to provide for greater efficiency in the use thereof in a particular environment.